1. Field of the Invention
The present invention relates to a shadow mask for a color cathode ray tube, and more particularly, to a shadow mask enjoying satisfactory luminance and white uniformity and having substantially rectangular apertures, a shadow mask printing negative plate used for the manufacture of the shadow mask, and a method for manufacturing the negative plate.
2. Description of the Related Art
In general, a color cathode ray tube comprises an envelope including a panel having a spherical surface, and a funnel joined integrally to the panel. A phosphor screen composed of three-color phosphor layers is formed on the inner surface of the panel. A shadow mask, which has a large number of apertures disposed in a specific pattern, is arranged inside the phosphor screen so as to face the same. Three electron beams, which are emitted from an electron gun located in a neck portion of the funnel, are deflected by a magnetic field generated by means of a deflection yoke, which is mounted outside the funnel. Thereafter, the electron beams are selected by means of the shadow mask so as to land properly in desired positions on the three-color phosphor layers. Then, the electron beams are scanned in the horizontal and vertical directions by means of the magnetic fields, whereby a color picture is displayed on the fluorescent screen.
Conventionally, the apertures of shadow masks of this type may be circular or rectangular in shape. Shadow masks having circular apertures are used mainly in display tubes, while ones having rectangular apertures are adapted principally for household use, such as home TV sets.
Conventionally, each aperture of a rectangular-aperture shadow mask is formed so that the direction of its longitudinal axis is in alignment with that of the vertical axis of the shadow mask. In particular, a plurality of apertures are arranged along the vertical axis, which passes through the center of the shadow mask, with narrow bridge portions between them, and a plurality of aperture trains, each extending in the direction of the vertical axis, are arranged side by side at predetermined pitches in the horizontal direction. The phosphor screen is provided with a plurality of trios of stripe phosphor layers corresponding to this shadow mask, each extending in the vertical direction.
A shadow mask having the apertures arranged in the specific pattern described above is manufactured by photoetching. More specifically, a sensitizing solution is applied to both sides of a mask substrate to form photo resist films, and a pair of shadow mask printing negative plates, having patterns corresponding to the apertures to be formed, are bonded individually to the photo resist films to effect printing (exposure) and development. Thus, resist patterns corresponding to the patterns on the negative plates are formed on the mask substrate. Thereafter, the mask substrate, having the resist patterns thereon, are etched from both sides, whereupon the shadow mask is completed.
The apertures of the shadow mask manufactured by this method are only approximately rectangular apertures they have four round corners, due to sagging of the patterns after the printing and development or difference in etching speed. Each of apertures in the negative plates used to print the patterns on the photo resist films however, has an accurate rectangular form and no roundness in its four corners. The etching method allowing substantially rectangular smaller openings having four round corners to be formed on one side of the mask substrate, while substantially rectangular larger openings having four round corners and communicating with the smaller openings are formed on the other side of the substrate. Each aperture is defined by the boundary between its corresponding smaller and larger openings. Projecting portions, which project toward the aperture, are formed at the boundary between the smaller and larger openings.
Generally, the shadow mask is arranged inside the panel in a manner such that the smaller openings are situated on the electron-gun side, and the larger openings face the phosphor screen. Therefore, those electron beams which irradiate the three-color phosphor layers at the central portion of the phosphor screen reach the screen after passing through the apertures at the central portion of the shadow mask in a direction substantially parallel to the axis of the apertures. However, those electron beams which land on the phosphor layers at the peripheral portion of the phosphor screen reach the screen after being positively deflected and diagonally traversing the apertures at the peripheral portion of the mask. Part of each electron beam thus diagonally traversing the apertures runs against the open edge portions (on the fluorescent-screen side) of the larger openings or inner aperture walls, and fails to reach the phosphor screen. Accordingly, luminous regions on the three-color phosphor layers which are formed corresponding to the respective configurations of the apertures are not rectangular, and have cutouts at the corners thereof. Thus, the luminance and white uniformity are lowered. Further, the beams reflected by the inner walls of the apertures may cause a different-color fluorescent layer to glow, thereby lowering the intensity of color or contrast.
In the case of an aperture which has the projecting portions at the boundary between the smaller and larger openings, in particular, the position for the formation of the projecting portions on the short-side portions of the aperture is shifted in the thickness direction of the mask from that of the projecting portions on the long-side portions, depending on the variation of the etching speed. Usually, the projecting portions at the short-side portions of the aperture are situated on the phosphor-screen side (on the side of the larger opening edge) of the ones at the long-side portions of the aperture. These projecting portions form stepped portions at the four corners of the aperture or the boundaries between the short- and long-side portions. More specifically, projecting portions situated on the phosphor screen side of the ones at the long-side portions are formed individually at the four corners of the aperture. If the electron beams diagonally traverse the apertures having these projecting portions, therefore, they are substantially intercepted by the outer corners of the apertures nearer to the outer peripheral portion of the shadow mask, so that the luminance and white uniformity are further lowered.
This problem is liable to arise, in particular, in the case of a flat square tube in which the panel has a substantially flat surface with a large radius of curvature. Namely, the radius of curvature of the shadow mask increases depending on that of the panel. In order to prevent the mechanical strength of the shadow mask from being lowered by the increase in the radius of curvature, the thickness of the mask must be increased. In the case of the shadow mask for the flat square tube, therefore, the electron beams diagonally traverse the apertures of the mask at a larger angle even though they deflect at the same deflection angle as in the case of use in a conventional color cathode ray tube. Thus, the electron beams are liable to run against the screen-side open edge portions of the apertures or inner aperture walls, so that the luminance and white uniformity are additionally lowered.
An off-center shadow mask is conventionally provided in order to prevent a cutout of each luminous region attributable to the collision of the electron beams which diagonally traverse each aperture. In the shadow mask of this type, the central portion of the mask has apertures formed so that the respective central axes of the smaller and larger openings are in alignment. As the peripheral portion of the shadow mask with respect to the horizontal direction is approached, the position of each larger opening is deviated outward with respect to its corresponding smaller opening. As the peripheral portion of the shadow mask with respect to the diagonal direction is approached, moreover, the position of the larger opening is deviated in the diagonal direction with respect to the smaller opening.
If the deviation of the larger opening with respect to the smaller opening is increased, however, the aperture configuration deforms. In the case of the flat square tube in which the panel has a substantially flat surface with a larger radius of curvature than that of the panel of a conventional color cathode ray tube, in particular, the radius of curvature of the shadow mask increases in proportion to that of the panel. As the size of the color cathode ray tube increases, therefore, the mechanical strength of the shadow mask considerably lowers, so that the shadow mask is expected to be relatively thick. In the shadow mask of this type, the electron beams which diagonally traverse the apertures run against the inner surface of each aperture, even though they do not in the case of the conventional shadow mask. Further, the aperture width as viewed from the path of the deflected electron beams is reduced, so that the luminous regions on the phosphor layers are narrowed, thus entailing lowered luminance. In order to avoid the collision of the electron beams and the lowering of the luminance, it is necessary only that the deviation .DELTA.W of the larger openings with respect to the smaller openings be increased. If the deviation .DELTA.W of the larger openings is increased, however, the height of each projecting portion on the right-side of each aperture is so different of that of each projecting portion on the left-side that the aperture configuration is further distorted.
Published Examined Japanese Patent Application No. 63-49336 discloses a shadow mask in which all the corners of larger and smaller openings are projected outwardly so that the openings are spool-shaped, in order to reduce the roundness of the four corners of each aperture. One embodiment in particular describes a version in which the difference in size between the larger and smaller openings with respect to the direction of the aperture width is equal to that with respect to the direction of the aperture length.
In order to prevent electron beams from being intercepted at the larger openings in the direction of the aperture width in the shadow mask constructed in this manner, however, the width of the bridge portions at the respective open edge portions (on the fluorescent-screen side) of the larger openings must be increased. As a result, the substantial width of the bridge portions at the projecting portions is increased. This lowers the luminance. In order to reduce the substantial width of the bridge portions, in contrast with this, the width of the bridge portions at the open edge portions of the larger openings must be reduced. As a result, the electron beams are intercepted to a higher degree at the open edge portions of the larger openings or projecting portions, so that the luminance and white uniformity are lowered. Moreover, if the construction of the shadow mask of this type is made similar to that of a conventional shadow mask in which the difference in size between the larger and smaller openings with respect to the direction of the aperture width is greater than that with respect to the direction of the aperture length, very large stepped portions are formed at the four corners of each aperture or the boundaries between the short- and long-side portions of the aperture. Accordingly, even though the shape of the aperture is rectangular as viewed from just above the aperture, the electron beams which diagonally traverse the apertures are intercepted by the stepped portions at the outer corners of the apertures nearer to the outer peripheral portion of the shadow mask, and luminous regions on three-color phosphor layers are subject to cutouts, so that the luminance and white uniformity are lowered.
Published Unexamined Japanese Patent Application No. 1-175148 discloses a shadow mask in which the corners of larger openings are projected outwardly so that electron beams can be prevented from being intercepted at the corners of apertures. This shadow mask, however, differs from the one disclosed in published Examined Japanese Patent Application No. 63-49336 only in the configuration of each larger opening, and the substantial shape of the apertures is same as that of the above Application, thus being subject to like problems.
Published Unexamined Japanese Utility Model Application No. 50-124253, moreover, discloses a shadow mask in which the central portion of each short side of each aperture is bulged inward so that the roundness of an end portion of an electron beam (cutout of each corner of a luminous region on a phosphor layer) caused by diffusion is eliminated to make the electron beam configuration rectangular. In the case of this shadow mask, however, if apertures are formed with bridge portions having a predetermined width left at the respective open edges of larger and smaller openings, the width of the bridge portions is so great that the luminance is low. In order to maintain the luminance level, however, the width of the bridge portions must be considerably reduced, so that the mechanical strength of the shadow mask with respect to the direction of aperture trains, each including a plurality of apertures arranged with the bridge portions between them, lowers. In press-molding the shadow mask into a predetermined shape, therefore, the mask undergoes local elongation or distortion. Thus, the desired shadow mask cannot be obtained.
Disclosed in Published Unexamined Japanese Patent Application No. 1-320738, furthermore, is a shadow mask in which larger openings are substantially rectangular, the outer corners of smaller openings are bulged, and the outer corners of apertures are also bulged so that electron beams diagonally traversing the apertures can be prevented from running against the open edge portions of the larger openings or inner aperture walls, and cutouts of luminous regions on three-color fluorescent layers can be prevented. Published Unexamined Japanese Patent Application No. 2-86027, moreover, discloses patterns of a shadow mask printing negative plate for forming those apertures. In this case, patterns corresponding to smaller openings are formed by combining rectangular main patterns and rectangular auxiliary patterns by composite exposure.
In the shadow mask of this type, although the white uniformity can be positively restrained from being lowered by cutouts of luminous regions, the roundness of the corners of apertures cannot be reduced, so that the luminance cannot be satisfactorily improved. At the outer peripheral portion of a phosphor screen where the allowance for electron beam landing is small, moreover, electron beams passing through bulging portions of the apertures are applied to fluorescent layers of different colors, and are liable to lower the color intensity.
Published Unexamined Japanese Patent Application No. 2-40840 discloses a shadow mask in which the four corners of each smaller opening are bulged outward, and the inner wall of the short-side portion of the smaller opening is slanted so that the roundness of the four corners of the aperture is reduced. In this shadow mask, however, the short-side portion of each aperture is arcuate and includes no straight portion, so that the aperture area is too small to obtain a satisfactory luminance. Since larger openings are arranged in the same manner as those of conventional apertures, moreover, the corners of the apertures cannot be easily bulged outward.
Published Unexamined Japanese Patent Application No. 55-159545 discloses a shadow mask printing negative plate whose apertures are I-shaped so that the four corners of the apertures are bulged outwardly. In a shadow mask formed by using the negative plate constructed in this manner, the outer corners of each aperture are bulged so that lowering of the white uniformity, which is caused by the collision of electron beams diagonally traversing the apertures, can be restrained in some measure. Since the apertures are formed so that larger openings are substantially rectangular and the corners of smaller openings are bulged outwardly, however, the roundness of the four corners of each aperture cannot be reduced, so that the luminance cannot be satisfactorily restrained from being lowered.
Published Unexamined Japanese Patent Application No. 56-156636 discloses a shadow mask printing negative plate in which each aperture has projecting portions sharply projecting for several tens of microns from its four corners individually. A shadow mask formed by using the negative plate constructed in this manner can be designed so that its apertures are each in the form of a rectangle having four corners with reduced roundness. However, no bulging portions are formed at the outer corners of the apertures against which electron beams are liable to run as they diagonally traverse the apertures. With these apertures, therefore, cutouts of luminous regions cannot be prevented, so that the white uniformity is lowered.
Although various improved shadow masks have been described above, their luminance and/or white uniformity can be improved only to some degree, and not satisfactorily.